1. Field of the Invention
The present invention relates to an aqueous dispersion, composite powder and an organosol of a fluorine-containing polymer.
2. Description of the Related Art
Organosol
An organosol of polytetrafluoroethylene (herein-after referred to as "PTFE") is known as a coating material which provides a coating film having good sliding and stain-proofing properties and corrosion resistance, and prepared by adding an organic medium having a boiling point higher than 100.degree. C. to an aqueous dispersion comprising colloidal particles of PTFE which is prepared by emulsion polymerization and then removing water from the mixture by heating (an azeotropic method. see Japanese Patent Publication No. 18775/1974).
Since the PTFE colloidal particles prepared by the emulsion polymerization are hardly redispersed once they are agglomerated, a large amount of a surfactant is added to the aqueous dispersion to prevent agglomeration.
A low molecular weight PTFE which is relatively less agglomerated is disclosed (cf. Japanese Patent Publication No. 31096/1972). To the same end, a copolymer of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (this copolymer being referred to as "FEP") which is less agglomerated than PTFE is used (cf. U.S. Pat. No. 2,937,156, GB Patent No. 1,094,349 and Japanese Patent Publication No. 17548/1973).
To prepare the organosol of PTFE, Japanese Patent Publication No. 17016/1974 discloses a phase transfer method which comprises adding PTFE to a phase transfer agent such as a water-soluble organic solvent or an aqueous solution of an electrolyte and stirring the mixture to transfer PTFE particles to a phase transfer liquid such as an organic solvent which is insoluble or hardly soluble in water. In this method, the FEP particles should be mixed while preventing agglomeration of the PTFE particles.
The coating film prepared from the aqueous dispersion containing a large amount of the surfactant tends to suffer from blisters or discoloration. The coating film prepared from the aqueous dispersion comprising FEP has inferior wear resistance so that its sliding property is greatly deteriorated during use.
In general, the colloidal particles of the high molecular weight PTFE having a molecular weight of, for example, about 1,000,000 or larger are easily fibrillated under small shear force, and fibrils are tangled together to agglomerate. Therefore, it has been impossible to form an organosol from the aqueous dispersion of colloidal particles of PTFE having a high molecular weight by the phase transfer method or by redispersion of the PTFE powder from the aqueous dispersion, which powder is often referred to as fine powder, in the organic solvent.
The formation of organosol of PTFE particles may be made easy through a decrease of the molecular weight of PTFE to several thousand to several hundred thousand by using a large amount of a chain transfer agent during polymerization or by irradiation of the fine powder. However, when PTFE is used for lubrication, the low molecular PTFE is worn heavily.
Blending of PTFE with, thermoplastic or thermosetting resins or elastomers
With utilizing good performances of PTFE such as heat resistance, flame retardance, non-tackiness, low friction, chemical resistance, low dielectric characteristics and the like, PTFE is used to modify other thermoplastic or thermosetting resins or elastomers through blending. Since PTFE has less compatibility with other resins or elastomers, PTFE particles are not sufficiently dispersed in a molded article of the blend. For a long period of time, it has been desired to increase the blending ability of PTFE without deteriorating the good performances of PTFE.
The low molecular weight PTFE which is a so-called "wax" is most widely used for blending. The PTFE wax can be blended with other polymers to some extent, while it has insufficient mechanical properties because of the low molecular weight. The aqueous dispersion or powder of high molecular weight PTFE may be used for blending but has poor dispersibility in other polymers so that dispersion in a submicron order can hardly be expected.
To produce a milky molded article of the thermoplastic or thermosetting resin or the elastomer, an inorganic white pigment such as quartz fine powder, alumina hydrate, aluminum hydroxide and calcium carbonate is added and mixed in the base polymer. Since the inorganic pigment is harder than the base polymer, it appears on the surface of molded article and deteriorates the surface smoothness when the pigment and the polymer are simply mixed. To keep the surface smoothness of the molded article, a special technique is required. When the amount of the inorganic pigment increases, the molded article itself becomes harder resulting in decrease of impact resistance.
Since PTFE powder is white and heat resistant, it is preferred as an organic white pigment which is a substitute for the inorganic white pigment. However, as described before, its addition and mixing is difficult.
As composite PTFE colloidal particles, Japanese Patent Kokai Publication No. 232109/1984 discloses composite particles each comprising a core of PTFE and a shell of a fluorine-containing elastomer, and Japanese Patent Kokai Publication No. 109846/1987 discloses composite particles each comprising a core of PTFE and a shell of a copolymer of TFE and chlorotrifluoroethylene (CTFE).
When the shell is made of the fluorine-containing elastomer, the composite particles have poor redispersibility in an aqueous medium or an organic solvent or poor ability to form the organosol. In particular, in the blending with another polymer, the sliding property of the molded article is deteriorated.
When the shell is made of the TFE/CTFE copolymer, the particles do not have good two-layer structures, and the molded article made from the blend of such composite particles and another polymer has poor wear resistance. It is disadvantage to use a pressure vessel for the polymerization of the core polymer from the view point of economy and apparatus.
Japanese Patent Publication No. 2099/1976 discloses coating of fluororesin particles with a vinyl base resin by seed polymerization of a hydrocarbon type vinyl monomer such as methyl methacrylate in an aqueous dispersion of the fluororesin, whereby electrophoresis of the particle is made easy so that smooth and good electrocoatin,g is achieved. However, when the hydrocarbon type vinyl monomer is polymerized in the aqueous dispersion containing colloidal particles of PTFE, affinity between PTFE and the hydrocarbon type vinyl monomer is poor, so that the PTFE particle surfaces are not coated with the vinyl monomer and new particles consisting of a polymer of the hydrocarbon type vinyl monomer tend to be formed. In addition, since the polymer of the hydrocarbon type vinyl monomer has a low thermal decomposition starting temperature, it should be removed from the electrocoated film before sintering by dipping the coated film in a hydrophilic organic solvent. Otherwise, the coated film tends to crack and no continuous film is obtained.
Japanese Patent Kokai Publication No. 284201/1988 discloses a process for preparing an organosol by polymerizing a fluoro(meth)acrylate in an aqueous dispersion comprising the fluoropolymer to modify the particle surfaces of the fluoropolymer with the fluoro(meth)acrylate polymer. However, since the disclosed fluoro(meth)acrylate polymer has a low glass transition temperature and has poor solvent resistance, the particles tend to agglomerate and fuse together in the organosol. Therefore, a polyfunctional internal cross linking agent should be copolymerized. When the aqueous dispersion is coagulated to prepare powder, the dispersibility of the powder is not good and the powder is hardly dispersed in the submicron order. In addition, since the thermal decomposition starting temperature of the fluoro(meth)acrylate polymer is usually from 200 to 250.degree. C., when the coated film of this polymer is sintered, it tends to crack.